Optical signals are increasingly being used for transmitting information signals. One impediment to an even greater use of optical signals is that optical devices and electro-optical devices are often substantially more complex and expensive to manufacture than their electronic counterparts. The term optical devices will be used in this disclosure to denote both optical devices and electro-optical devices. This is because optical devices typically include a number of optical components that have to be precisely aligned relative to one another. Moreover, the optical devices have to be designed and manufactured so that their constituent optical components, once aligned, stay in alignment despite changes in environmental factors, such as temperature, vibration, etc.
One common optical device is the optical receiver that receives light, typically via an optical fiber, and detects the light to generate an electrical information signal that represents the information signal conveyed by the light. An optical receiver typically includes an input fiber, a converging optical element and a light detector. These components must have consistent optical and physical characteristics and must be accurately physically aligned relative to one another to ensure that the optical signal received via the optical fiber falls on the light detector. The need for the optical components to have consistent optical and physical characteristics and the need to align the optical components accurately relative to one another make a conventional optical receiver expensive to manufacture.
The component characteristics and alignment tolerances may be relaxed by increasing the area of the light-sensitive surface of the light detector. However, increasing the area of the light-sensitive surface increases the capacitance of the light detector. The capacitance of the light detector determines the maximum frequency of the information signal that can be recovered from the optical signal. A light detector that has a small enough capacitance to recover an information signal in the GHz frequency range may not have a large enough light-sensitive surface to allow the component characteristics and alignment tolerances to be significantly relaxed. Also, portions of the detector not illuminated by the optical signal may generate sufficient noise to degrade the signal-to-noise ratio of the electrical information signal output by the light detector.
The optical signal received via the optical fiber may be a multi-frequency optical signal, such as a wave-division multiplexed (WDM) optical signal or a dense wave-division multiplexed (DWDM) optical signal. In this case, the optical receiver typically additionally includes a frequency-dispersive element, such as a diffraction grating, and a light detector for each optical frequency included in the multi-frequency optical signal. The light detectors may be configured as a detector array having one detector element for each optical frequency. An optical receiver for a multi-frequency optical signal has component characteristics and alignment tolerances at least as severe as those of an optical receiver for a single-frequency optical signal. Morever, it is more difficult to use a large-area light detector in an attempt to relax the component characteristics and alignment tolerances in such an optical receiver.
Therefore, what is needed is an optical receiver having relaxed component characteristic tolerances and relaxed alignment tolerances and that is capable of detecting a high-frequency information signal conveyed by the optical signal. What is also needed is such an optical receiver that is additionally capable of generating an electrical information signal having a high signal-to-noise ratio. Finally, what is also needed is such an optical receiver that is additionally capable of receiving a multi-frequency optical signal and generating an information signal from each of the optical frequencies.